This invention generally relates to the formation of gas tight pressure blocks in multiple wire or conductor electric cables such as telephone cables to enable air pressurization of sections of such cables. In particular, this invention is useful in maintaining fluid pressure in the pressure block forming compound during multiple injections at a single site on a cable during in-field service or installation.
U.S. Pat. No. 3,823,250, the disclosure of which is incorporated herein by reference, discloses a tap fitting suitable for use with the valve hereinafter described. Briefly, the tap fitting comprises a curved base sheet for placement over an opening cut in the sheath of the cable and an injection port integrally formed with and projecting from the sheet, the port including means for forming a pressure tight covering, such as threads about the mouth of the port.
As is well known in the art, air pressurization of sections of electric and telephone cables prevents the seepage of water or moisture into the cable and the resulting noise and interference in communication and power circuits. Pressure blocks forming dams at intermittent points over the length of the cables are provided by injecting a self-setting liquid compound which hardens and fills the space around the wires over a sufficient length to hold the air pressure. To form such a pressure block it is necessary to make an opening in the cable sheathing leading to the interior and then to provide and exteriorly positioned fitting through which the liquid block forming compound may be injected. To accomplish this, a fluid injector is used. Typically the fluid injector comprises a chamber containing a discrete amount of block forming compound, a pressurizing plunger operative in the chamber, and a nozzle in fluid communication with the chamber through which the fluid is ejected in response to the compression movement of the plunger operating on the compound. The fluid compound is injected into the interior of the cable by threading the nozzle of the fluid injector to the tap fitting and compressing the plunger.
Where a large amount of compound is needed to fill the cable interior to the desired extent, i.e., more than the amount normally contained within the injector chamber, the continuous injection operation must be interrupted so that the injector chamber can be refilled or recharged. To recharge the fluid injector in the midst of the injection operation, the pressure on the plunger of the fluid injector is released and the plunger is withdrawn to permit access to the injector chamber for refilling with the compound.
Recharging in the midst of the cable filling for block forming operation presents a number of drawbacks. Due to residual pressure in the cable, some of the liquid compound escapes through the tap fitting and pressure interior of the cable is normally lost. To overcome these problems, stop valves have been devised to be interposed between the fluid injector and the cable interior. Such stop valves permit injection of fluid through a fitting into the cable interior while opposing the escape of the pressurized fluid compound therefrom. The 3M Company of Minneapolis, Minn., for example, markets a resin pressure splicing system under the trademark "SCOTCHCAST Brand R.P.S." The 3M system includes a one-way valve that opens to permit the injection of the blocking compound and closes under the residual back pressure to prevent the escape of the blocking compound.
Such prior art valves have considerable drawbacks, particularly where the nozzle of the fluid injector is adapted to sealably couple with a conduit (which includes the valve) into the cable interior. For example, due to the pressure seal between the injection nozzle and the valve, it is difficult to release the plunger of the fluid injector, since any attempt to do so creates a vacuum in the interior of the fluid injector. This makes it difficult to recharge the fluid injector while it is in place on the tap fitting.